1. Field of Invention
The present invention generally relates to the steering of waterjet propulsion systems and, more particularly, to a steering and reversing gear designed to be integral with a marine vessel hull, i.e., entirely independent of the waterjet pump and nozzle, and suitable for use with high-powered waterjet propulsion systems for large vessels.
2. Brief Description of Related Art
Early waterjet propulsor applications were particularly for use on small boats and recreational vehicles such as jet skis. Since about 1980, waterjet propulsors have become increasingly used with larger and larger vessels. As a result, the pumps and steering and reversing gear of large waterjet propulsion systems have become increasingly larger and have been exposed to increasingly larger stresses. The attachment of such large propulsion systems to large vessel must be relatively robust in order to carry thrust loads and to maintain hull integrity in the event of contact with underwater objects. Consequently, such applications of waterjet propulsion systems with large marine vehicles requires very large pump housings to withstand the applied loads. However, as the size and power of waterjet propulsors increase, the problems associated with the provision of steering and reversing gear increase in difficulty. In large part this is due to the familiar squared-cubed law, whereby masses, and hence weight and dynamic force, increase with the cube of the scale factor, whereas cross-sections available to resist these forces increase only with the square of the scale factor, while material properties remain the same.
The steering and reversing gear of known, commercially available waterjet propulsion systems are mounted to and supported by the pump and, thus, transmit large forces to the pump housing. For small and medium sized waterjets, this arrangement presents no particular problem. The pump body is robust for other reasons and only local strengthening is needed to take weight and hydrodynamic forces arising from steering and reversing gear. However, these forces are not small since they include the following: steering forces up to about half the maximum gross thrust; reverse forces up to about 1.5 times the maximum gross thrust; dynamic forces due to ship motions; wave loads due to slamming; impact loads during docking, etc.; and weight of the steering sleeve or nozzle, reversing bucket, actuators and entrained water. All these forces are reacted at the pump body attachment points and are transmitted into the hull through the pump mounts whether the pump is transom mounted or otherwise.
As the size of the waterjet is increased to accept higher powers, the material thickness must increase disproportionately since, as stated earlier, weights and forces tend to increase as the cubed of the scale factor while available material sections increase only as the square of the scale factor. However, increasing material thickness to hold down stresses results in more weight and higher dynamic forces associated with weight. A point is eventually reached where it is no longer possible to maintain maximum stresses within safe limits for materials of choice. The designer then has two options; use higher strength materials, which are costly, or consider a different design concept. It may not be sufficient to make the pump body stronger since its attachment to the hull must also be considered.
More recently, there has been an interest in applying waterjet propulsion to large Naval vessels, e.g., large sea-going displacement vessels such as Naval Destroyers, because of the efficiencies which may be achieved over more conventional propulsion systems. The U.S. Navy recently initiated a study to explore the application of waterjets to surface ships having a design speed of about 30 knots and requiring twin propulsors with power in the range of 50,000 hp (37 mW) per propulsor. However, such arrangements are not easily scaled to sizes usable in larger sea-going vessels. With a proposed nozzle equivalent diameter approaching 3 meters, the size and weight of a conventional steering and reversing gear are impractical for attachment to the pump body. In addition to disproportionate increases in strengthening the mounting structure to carry increased thrust loads, the control linkages for rotating the waterjet propulsor must be greatly enlarged. Even in the case of waterjet propulsors for small craft, control linkages are a significant portion of the weight of the entire propulsion system. With larger propulsors the weight becomes a much larger fraction of the total weight of the system.
Further, the control linkages would also be outside the vessel and constitute a much more serious source of drag and are far more vulnerable to damage. The manipulation of such large structures would be accomplished hydraulically and damage to a hydraulic hose or other damage could easily disable a larger vessel, leaving at most, only differential thrust from propulsors displaced from the vessel centerline for steering and maneuvering the vessel. Stresses in the control linkages and bearings would also become very high in large waterjet propulsion systems and could lead to fatigue and failure.
Additionally, reverse thrust presents special problems as waterjet propulsion systems are made larger. Specifically, while the entire waterjet propulsor may be rotated 180.degree. to obtain reverse thrust on a small propulsor, this cannot readily be done with a large system. For this reason, on large waterjet systems where the pump is inside the hull, steering is generally accomplished with a conventional steering sleeve or nozzle attached to and supported by the pump. Reverse thrust is obtained with a conventional reversing bucket integral with the steering sleeve.
In summary, in available designs, all loads on the steering and reversing mechanisms, and other parts of the propulsion system due to motion of the vessel as well and thrust forces, are transmitted to the vessel through the propulsor pump. Known mechanisms for producing reverse thrust are also required to be sufficiently robust so that, for a large vessel, the weight thereof represents a significant source of inefficiency. To date, no mechanical system for accomplishing steering, braking and reversing in large waterjet propulsion systems, i.e., propulsors having power greater than about 27,000 hp (20 mW), has been proposed which acts independent of the waterjet propulsor, e.g., is mounted entirely independent of the pump, in order to minimize transferred loads on the propulsion system. Thus, there is a need for a steering and reversing gear that overcomes the problems associated with pump supported steering and reversing gear to be installed on large vessels such as Naval Destroyers.